Q: What could cause these stains? Bleach and a pressure washer will not clean it off.
A: It sure looks like the stains caused by pendimethalin pre-emergent. The label clearly says to avoid getting it on hard surfaces. The chemical itself is an analine dye so it’s really hard to remove.
On concrete, one option is to apply muriatic acid and gently work it in with a brush. The acid will dissolve the concrete just enough to get rid of the stain. On a hard surface, try citrus-based automotive hand cleaner or brake cleaner. If they don’t work well enough, gently rub auto paint restorer into the stain.